They’re coming, and in many hospitals, they have already arrived: multidetector CT (MDCT), CT angiography (CTA), and advanced 3D imaging. This wonderful new modality offers the promise of evaluating disease processes from atherosclerosis to cancer to trauma more quickly, safely, and accurately than older techniques can. CTA is already replacing diagnostic catheter angiography in many institutions. The emergency evaluation of chest pain is changing rapidly, with MDCT used to evaluate the coronary arteries, pulmonary arteries, and aorta (often in a single breath hold), while at the same time allowing adjacent soft tissues to be seen for assessment for lung disease, chest-wall abnormalities, and upper-abdominal disease.
Complex fractures are being evaluated in 3D for presurgical planning, with the anticipation of shorter, smarter surgical procedures and improved outcomes. Complex organs like the liver and pancreas can now be evaluated in detail prior to surgery, allowing surgeons to define optimum dissection planes for resection of disease. CT perfusion is being used more and more in the acute setting for stroke, in order to guide patient management in the crucial early hours after the onset of symptoms. New applications for this technology are being developed every day. It is clear that advanced visualization has moved from being a luxury to being a necessity in the practice of medicine.
This article will focus on ways to maximize radiologist efficiency in processing and interpreting CTA studies. In the world of 3D, CTA is far and away the highest-volume procedure. It is critical, therefore, to develop both the skill sets and the workflow processes to handle ever-expanding volumes of cases as efficiently as possible. For the time and effort involved, professional reimbursement for CTA is low, generally being about $15 to $30 more than for a conventional CT. If it takes four times as long to read a CTA of the carotids than to read a CT of the neck, you are losing money. How, then, can we, as radiologists, offer high-quality CTA to our patients and referring physicians without getting bogged down in workflow inefficiencies? There are four key steps to maximizing efficiency in CTA: knowledge, support, access, and routine.
Radiologists interpreting CTA must become familiar with the reconstruction algorithms commonly used in advanced visualization. The four most common reconstruction processes are 3D volumes, multiplanar reconstruction (MPR), maximum intensity projection (MIP), and curved planar reformats (CPRs).
3D volumetric images are created by taking the original CT axial slices and finding common boundaries based on Hounsfield units. These boundaries are converted into shaded surfaces to create 3D structures. Different structures can be brought out by varying the Hounsfield unit thresholds, and artificial-intelligence algorithms can be applied in order to segment out certain anatomic structures of similar density. As an example, programs have been written to recognize and remove the chest wall in coronary CTA automatically, or to remove the bones in a lower-extremity runoff CTA.
3D volumetric imaging provides an excellent overview of the spatial relationship between structures and offers a guide for defining other optimal reconstructions. The disadvantages of 3D volumetric images are that the resolution is low and heavy calcium can overwhelm the image and obscure underlying blood vessels. Typical uses of 3D volumes are bypass-graft orientation in coronary CTA, defining the relationship of the carotid bifurcation to the angle of the mandible in carotid CTA (Figure 1), and evaluating and comparing the degree of angulation across the neck of an abdominal aortic aneurysm.
MPRs are the backbone of CTA interpretation for many anatomic regions. With the advent of MDCT, slices have become so thin that voxels are now isotropic. In other words, the voxel has become cubic in shape. This means that images can be reconstructed in any plane with no loss of resolution, and none of the annoying stair-step artifacts that we used to deal with in the early days of CT. For CTA, MPRs are extremely useful for evaluating stenoses, as well as for evaluating the superior and inferior ends of curved structures like blood vessels and organs. The main disadvantage of MPR is that